I’m getting a hum in my integrated tube amp and it’s bugging me alot

Hi - first of all, here’s my system:

- Rogers High Fidelity EHF-200 mkII integrated tube amp

- Magico A5 speakers

- Merging NADAC + Power DAC

- Audience T2 power conditioner

- Purist cabling

I love the sound of my system. BUT…….I have been noticing a hum after about 10 minutes of use from my amp. There are two switches on the amp - POWER & OPERATE - that need to be turned on in order to listen to music. (The Power switch needs to be turned on about 30 seconds prior to switching on the Operate switch). With both KT150 and KT88 tubes (all matched), after about 10 minutes I hear an audible hum from the amp that is apparent from my listening position about 10 feet away. If I turn off the OPERATE switch, the hum disappears (but of course, then I can’t listen to music). I have contacted Roger (the company owner) who said it was noise from old tubes. But my KT88’s have less than 100 hours on them and they hum as well as the KT150’s (maybe a few db’s softer than the 150s’). 

Does anybody have any ideas about what causes this and how it might be fixed? Is this an issue that is endemic to tube amps? FYI - if this requires new tubes, then I need to switch back to SS as I’m not willing to invest approx $800/year in new tubes. 

I turn to the great wisdom of this forum to help.

Thanks - Matt